Ch_322a_8.07

Ch_322a_8.07 - Addition of Bromine and Chlorine to Alkenes...

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Addition of Bromine and Chlorine to Alkenes Alkanes do not react with Br 2 or Cl 2 in the absence of ultraviolet or visible light at room temperature. Alkenes r apidly undergo additions reactions with Br 2 and Cl 2 in the dark (no radiation) at room temperature: C=C + X 2 dark 25 o C C-C X X The Bromination Reaction as a Diagnostic Test for Alkenes and Alkynes C=C + Br 2 red-brown CCl 4 C C Br Br colorless C C + Br 2 red-brown CCl 4 C=C Br Br colorless
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Examples of the Halogenation Reaction CH 3 CH 2 CH=CH 2 1-butene + Cl 2 - 9 o C CH 3 CH 2 CHClCH 2 Cl 1,2-dichlorobutane Chlorination + Br Br Br Br H H H H trans- 1,2-dibromocyclopentane (Racemic Form) cyclopentene + Br 2 - 5 o C CCl 4 /ethanol Bromination Note: Only the trans -1,2-dibromocyclopentane is formed. This result is understandable when the mechanism of this reaction is examined.
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A Mechanism for the Bromination Reaction The proposed mechanism for bromination of an alkene follows the general two-step mechanism of addition. The first step is electrophilic
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Ch_322a_8.07 - Addition of Bromine and Chlorine to Alkenes...

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